The costs associated with monitoring subsurface conditions, such as the contents of groundwater (including monitoring well installation, sampling, analysis, and decommissioning), are often expensive and thus reduce resources that would otherwise be applied to other endeavors such as prevention and remediation. In addition, the data collected from monitoring wells is often difficult to interpret due to obstacles in obtaining a representative sample and the infrequency of sampling.
Conventional methodology is problematic for several reasons. For instance, standard operating procedure calls for the removal (purging) of 3–10 casing volumes of water from the monitoring well before collecting the sample. The amount actually collected for analysis is typically 1/1000 th or less of the total volume of water removed from the well. The purging process, designed to ensure that the sample is representative of the formation, is slow and cumbersome. Furthermore, the water removed sometimes must be treated as a hazardous waste, thereby incurring even more expense. Data quality due to loss of volatile analytes is also a serious concern because the sample is subjected to many procedures prior to analysis. These procedures include, for example, transfer to a container, transport to the off-site laboratory, storage, sample dilution, and clean-up or preconcentration. The data quality problem is especially acute for low contaminant concentrations, such as less than 50 μg/L of contaminant.
Improvement has been made in groundwater sampling. For instance, U.S. Pat. Nos. 5,804,743 and 5,996,423 describe the use of sampling bags made of semi-permeable membrane materials to collect volatile organic components in groundwater. The bags are lowered into a monitoring well, and the volatile organic components in the water allowed to permeate through the membrane into a reference fluid contained in the bags. The sampling bags are then withdrawn from the well and the fluid contents of the bags are analyzed for the presence of the dissolved volatile components.
Placing a sampling apparatus within a well eliminates the need to pump water to the surface for study and thus simplifies sampling. However, several problems are associated with this approach. For example, volatiles may escape from the water column or soil matrix into the headspace of the well, thereby reducing the amount of volatile substance available to achieve an equilibrium value between the inside and outside of the sample bag. In addition, when the sampling bag is removed from a well for analysis, some losses of volatile components may occur in the transfer of sample from the sampling bag to a sample container for subsequent transport to a laboratory.
Improved methods and systems for monitoring subsurface conditions are therefore needed.